The defending champion, John McGlashan College, reclaimed top spot with a thumping 96-0 win against St Kevin's College in Dunedin.
St Kevin's kept itself in the hunt for a spot in the top six with a win against South Otago High School a week earlier but the crushing defeat left it stuck in seventh place.
Waitaki Boys' High School beat Taieri College by default and has secured second place.
But Otago Boys' High School leapfrogged South Otago into third place with a 32-27 win against its rival in Balclutha.
Kavanagh registered its second win with a 38-17 victory against the winless Mt Aspiring College at Kettle Park but both sides were out of contention.
King's High School picked up a four-try bonus-point win with a 24-19 victory against Dunstan High School but both sides have claimed the final two places.
King's led Dunstan 19-7 at the break but had to weather a fightback in Alexandra.
Dunstan drew level but, with about five minutes left, King's fullback Josh Duff scored the winning try in his 25th game for the school's first XV.
Halfback Nicky Peat made a break, dashing up field 25m and King's swung the ball to the left and Duff went over.
He made a hero of himself at the other end shortly after when he pulled off a crucial tackle about 5m from the line and came up with the ball.
"They clawed their way back and put a lot of pressure on us,'' King's coach Kane Jury said.
"We just sort of had to defend and show a lot of ticker towards the end.
"[That strip] gave us about two more minutes with the ball.''
James Te Pairi, Dylan Tutty and Jack Clement all scored for King's in the first half.
